LBC Cross Talk

Description

The crosstalk (or XTalk) typically
occurs during the simultaneous readout of multiple
amplifiers. When multiple amplifiers are readout, such as occurs when
using multiple amplifiers in a single CCD or multiple CCDs in a
mosaic, the controller electronics relate the signals from pairs of
extensions in multiextension format (typically pairs of CCDs in raw
mosaic exposures, called "victim" and "killer", respectively).

The crosstalk causes pixel values produced by one amplifier to be
affected by the signal in another amplifier. For this reason, the
victim and killer pixels must be matched in "amplifier coordinates"
(given by the ATM keywords).

Using a simple model for the crosstalk, the signal for a pixel in one
amplifier, which we call the "killer", adds or subtracts a small
amount to the pixel value read at the same time in another amplifier,
called the "victim". A correction is obtained by multiplying the pixel
value of the killer image by a crosstalk coefficient and adding or
subtracting it from the matching pixel in the victim image.

Note that it is possible that a killer may also be a victim and that a
victim may be affected by multiple killers. In our simple model each
pair of killer and victim are treated independently and the killer
pixel values used to correct a victim are treated as unaffected by
other amplifiers.

The crosstalk correction should be performed before any other
operation. The simple model of the crosstalk is that the raw data from
the amplifier readout is used. Therefore the correction should
generally be applied only to the raw data.

Correcting the Cross Talk

The victim pixels of each extension must be corrected taking into account
the contribution of all the corresponding pixels in the killer extensions:

corrected = victim - xtcoeff * (killer1 + ... + killerN)

where the arithmetic is done on each pixel in the victim image extension
with the matching pixels in the killer image extensions and the scale
is the numerical coefficient xtcoeff. For LBC the xtcoeff is -1.83E-4.